What is the California Bearing Ratio (CBR)?
The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR @ 100%).
It is primarily intended for, but not limited to, evaluating the strength of cohesive materials having maximum particle sizes less than 19 mm (0.75 in.) (AASHTO, 2000[1]).
The CBR was developed by the California Division of Highways circa 1930 and was subsequently adopted by numerous states, U.S. federal agencies and internationally. As a result, most agency and commercial geotechnical laboratories in the U.S. and Canada are equipped to perform CBR tests.
The basic CBR test involves applying load to a small penetration piston at a rate of 1.3 mm (0.05″) per minute and recording the total load at penetrations ranging from 0.64 mm (0.025 in.) up to 7.62 mm (0.300 in.).
Figure 1.0 is a sketch of a typical CBR sample.
FIGURE 1.0
Equation
Values obtained are inserted into the following equation to obtain a CBR value:
Typical Values
Table 1.0 summarizes typical CBR values by soil type.
TABLE 1.0
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